In a first display period of a display device, a first count value is recorded at the rising edge of the data enable signal for controlling the length of a horizontal line. Next, a second count value is recorded at the falling edge of the data enable signal for identifying the time when the data enable signal switches from a high level to a low level. When entering a porch period following the first display period, the counter is cleared when the count value reaches the first count value. In a second display period following the porch period, the counter is cleared at the rising edge of the data enable signal, and the first count value is used for controlling the length of the horizontal line.